1. Field of the Invention
The present invention relates to a device for shielding an electronic circuit for aircraft, which is applied to a video monitor installed in aircraft.
2. Description of the Related Art
Of various types of aircraft, there are passenger planes installing video monitors (hereinafter referred to as xe2x80x9cmonitorxe2x80x9d) for explaining safety equipment and showing programs such as movies to passengers. While projectors or CRTs (cathode ray tubes) have been mostly used as the conventional monitors, liquid-crystal panels with improved sharpness are gaining popularity in recent years. When a liquid-crystal panel is used as the monitor, the screen is necessary to be made smaller than that of the conventional projector or the like, which needs also its electronic circuit for showing images to be made compact.
The electronic circuit of the electrical equipment includes components that radiate electromagnetic waves. Since the radiation of electromagnetic waves may, in some cases, give damage to the manipulating equipment of the aircraft, the components must be shielded so that the electromagnetic waves should not leak outside. By the way, when the electronic circuits are made compact as described above, it is difficult to shield the components, and hence the whole electronic circuits are to be shielded.
How this is implemented will be described. FIG. 11 is a block diagram showing a video display unit, which comprises a chassis plate 1, a switching power supply 2, a video signal processing circuit 3, an LCD display 4, a system control circuit 5, a motor 6, a clutch 7 and a brake 8 or the like. These components are connected as shown in FIG. 11. The chassis plate 1 has an ON/OFF signal input terminal la and a video signal input terminal 1b. The ON/OFF signal input terminal la is connected to the system control circuit 5, and the video signal input terminal 1b to the video signal processing circuit 3.
In this video display unit, when an ON signal is inputted to the ON/OFF signal input terminal 1a, the system control circuit 5 operates the switching power supply 2, so that predetermined power supply voltage is supplied to predetermined components. Then, when a video signal is inputted to the video signal input terminal 1b, the video signal processing circuit 3 is operated, so that an image is displayed on the LCD display 4. The motor 6 drives the LCD display 4 between the storage position and the display position. The clutch 7 and the brake 8 serve as auxiliary mechanisms for moving the display 4.
A CPU incorporated in the system control circuit 5 performs video display control as well as the whole control over the driving of the LCD display 4. The power supply voltages include a logic voltage supplied from the switching power supply 2, 5 V for operating the video signal, 24 V for supplying power to the clutch and the brake, and ON/OFF signals (28 V) from the aircraft body.
The CPU and a clock oscillation circuit could be the source of noise (electromagnetic waves) in the system control circuit 5. In the arrangement shown in FIG. 11, the electromagnetic waves generated from the CPU and the clock oscillation circuit are radiated to the pattern and the wiring of the printed board of the system control circuit 5 and emitted as electromagnetic waves. In order to suppress such emission of electromagnetic waves, a metal plate 13 may be used to cover a printed board 9, and a CPU 10, a clock oscillation circuit 11, wiring cables 12 and the like that are attached to the board 9 as shown in FIG. 12. However, if the covering metal plate 13 is connected to a circuit ground (ground circuit on the board) S G as shown in FIG. 13, these components covered by the metal plate 13 are not completely shielded, thereby leaking electromagnetic waves E. To overcome this inconvenience, the metal plate 13 is connected to a frame ground (ground circuit for the whole circuit) F.G, so that satisfactory shielding effects are obtained.
The connection of the metal plate 13 to the frame ground F.G will be described. The ground (circuit ground) for the output voltage (5 V and 24 V) of the switching power supply is connected through a capacitor (1000 pF), without being directly connected to the frame ground F.G. The reason is that the switching noise current flowing into the chassis (frame ground) is increased to thereby increase electromagnetic radiation and noise terminal voltage. Therefore, the circuit ground is not actually grounded for the circuit, and hence, when the metal plate is connected to the circuit ground under this condition, the potential of the metal plate is not grounded, either. That is, as shown in FIG. 13, the electromagnetic waves E are emitted from the metal plate 13. Noise does not propagate if the electromagnetic waves E are contained within the circuit so as not to be emitted outside as much as possible. Consequently, this technique is not appropriate.
As another technique for suppressing the emission of electromagnetic waves, the metal plate 13 may be connected to the frame ground F.G that is the stablest potential in the system as shown in FIG. 14. However, when many wirings are connected to the unit, it is not possible to cover the whole unit with the metal plate 13 structurally. Further, even when the unit is completely sealed, heat radiation effects are deteriorated, and hence this technique is not satisfactory, either.
The present invention has been made in view of these circumstances, and an object of the present invention is therefore to provide a structure in which metal plates are used to cover only components to be shielded, i.e., a CPU and a clock circuit that radiate electromagnetic waves generated in an electronic circuit and in which the metal plates are connected to the chassis (frame ground).
According to a first aspect of the present invention, there is provided a device for shielding an electronic circuit for aircraft, wherein a case formed of metal plates covers the outer circumference of a component to be shielded which radiates electromagnetic waves in an electronic circuit of electrical equipment to be installed in an aircraft, and part of the case is connected to a frame ground.
According to a second aspect of the present invention, in a device for shielding an electronic circuit for aircraft according to the first aspect of the invention, the case is a box-like member formed by combining a first metal plate that is U-shaped by bending both end portions thereof upward to form erect portions and a second metal plate that is U-shaped by bending side portions thereof downward to form erect portions, in such a manner that a middle portion of the first metal plate is placed below the component to be shielded and the erect portions on both end portions of the first metal plate are projected outside both end portions of the component to be shielded and a middle portion of the second metal plate is placed above the component to be shielded and openings at both side portions of the first metal plate are covered with the erect portions on both side portions of the second metal plate.
According to a third aspect of the present invention, in a device for shielding an electronic circuit for aircraft according to the first aspect of the invention, the case is formed by combining a first metal plate that is a flat plate having projections on both end portions thereof and a second metal plate that is a box-like member having four sides of a flat portion thereof bent downward so as to have only one opening on the bottom thereof, in such a manner that the first metal plate is placed below the component to be shielded and the second metal plate covers the component to be shielded from above.
According to a fourth aspect of the present invention, in a device for shielding an electronic circuit for aircraft according to the second or third aspect of the invention, the middle portion of the first metal plate has a width substantially equal to or smaller than a width of the component to be shielded.
According to a fifth aspect of the present invention, in a device for shielding an electronic circuit for aircraft according to the second or third aspect of the invention, the first metal plate and the second metal plate are combined by clamping part of one of the metal plates with an elastic force of the other metal plate.
According to a sixth aspect of the present invention, in a device for shielding an electronic circuit for aircraft according to the second or third aspect of the invention, the first metal plate and the second metal plate are combined by engaging recess portions or holes formed in part of one of the metal plates with projections formed on the other metal plate.
According to a seventh aspect of the present invention, there is provided a device for shielding an electronic circuit for aircraft, wherein an electronic circuit of electrical equipment to be installed in an aircraft is formed of a plurality of laminated printed boards, and components to be shielded that radiate electromagnetic waves in the electronic circuit are arranged on intermediate ones of the plurality of printed boards so that uppermost and lowermost ones of the plurality of printed boards cover the components to be shielded, and at least part of one of the uppermost and lowermost printed boards is connected to a frame ground.
According to an eighth aspect of the present invention, there is provided a device for shielding an electronic circuit for aircraft, an electronic circuit of electrical equipment to be installed in an aircraft is formed on the outermost ones of a plurality of laminated printed boards, and components to be shielded that radiate electromagnetic waves in the electronic circuit are arranged on the outermost printed boards, and ones of the plurality of printed boards interposed between the outermost printed boards incorporating the electronic circuit therein are connected to a frame ground.
According to a ninth aspect of the present invention, there is provided a device for shielding an electronic circuit for aircraft, an electronic circuit of electrical equipment to be installed in an aircraft is formed of a printed board, and a component to be shielded that radiates electromagnetic waves in the electronic circuit is covered with the metal plates attached to both surfaces of the printed board.
According to a tenth aspect of the present invention, there is provided a device for shielding an electronic circuit for aircraft, an electronic circuit of electrical equipment to be installed in an aircraft is formed of a printed board, and metal plates serving also as a heat sink are arranged on one surface of a component to be shielded that radiates electromagnetic waves in the electronic circuit so as to come in contact with the surface of the component.
As the structures provided by the first to sixth aspects of the present invention, when the outer circumferences of components to be shielded, such as a CPU and a clock circuit that radiate electromagnetic waves, are covered with a case made of metal plates and part of the case is connected to the frame ground, the radiation of electromagnetic waves outside can be suppressed reliably. Further, as the structures provided by the seventh and eighth aspects of the present invention, when a plurality of printed boards are used and part of such printed boards is connected to the frame ground, the leakage of electromagnetic waves outside can be likewise prevented. Still further, according to the ninth aspect of the present invention, the metal plates attached to both surfaces of the printed board shield the component to be shielded completely, and according to the tenth aspect of the present invention, the shielding and heat radiation for the heat radiating component to be shielded can be implemented at the same time.